This invention relates to roller tappets for use in internal combustion engines.
Roller tappets have been employed to increase engine breathing since they allow increased lift velocity without an increase in tappet body diameter. Roller tappets are disposed within tappet bores defined by an engine block. A roller on the body is engaged directly by a cam lobe on a cam shaft. Conventional roller tappets include a central valve lifter body having a lower bifurcated end. The bifurcated end is defined by a pair of depending legs. A roller is rotatably supported on a pin extending through the depending legs. The roller tappet bore of the engine is dimensioned to provide sufficient guide surface area for proper operation of the tappet.
While conventional, non-roller type hydraulic tappets are fabricated normally from hardenable or chilled gray iron, such materials are not strong enough for use with roller tappets. The legs of the bifurcated end of the roller tappet must have sufficient strength to withstand the loads imposed on them during operation. As a result, the industry has heretofore typically specified SAE 1144 steel for the fabrication of conventional roller tappets. Such material was felt necessary to provide sufficient strength and wearability for the roller tappet to have an adequate service life. Typically, a roller tappet body is machined from bar stock. This method of manufacture necessarily requires a large capital expenditure for machining equipment necessary to provide production requirements and to maintain the necessarily exact dimensional control. An example of a roller tappet of this type may be found in commonly owned U.S. Pat. No. 3,977,370, entitled Roller Tappet and issued on Aug. 31, 1976, to Humphreys.
Roller tappets fabricated from SAE 1144 steel experience problems with wear of the guide surfaces of the tappet bores. The steel tappet bodies have a tendency to gall or chafe within the roller tappet bores which increases tolerances and results in improper tappet operation.
Commonly owned U.S. Pat. No. 4,094,279, entitled Ductile Iron Roller Tappet Body and Method for Making same and which issued on June 13, 1978, to the present inventor discloses a roller tappet which alleviates the aforementioned problems. The roller tappet disclosed therein includes a central valve lifter body fabricated from nodular iron. The tappet body provides compatability with the cast iron engine block and substantially eliminates the heretofore experienced wear and galling problems. Fabrication from ductile iron also results in significant manufacturing advantages including increased tool life, reduced chip size, increased feed rates and increased ease of machinability when compared to tappets fabricated from SAE 1144 steel. The tappet is disclosed in my aforementioned patent as preferably being machined from bar stock material.
The nature of the environment of use of roller tappets, the forces and loads imposed thereon and prior design practices clearly dictate against use of weaker steels, or other materials, such as aluminum or cast iron. The present tappet constructions would not have sufficient strength if fabricated from such materials to withstand the loads imposed upon them in operation.